Free-space optical communication systems are capable of transmitting data at very high data rates over long distances. Acquisition schemes and precise beam pointing and tracking capabilities are required to communicate between moving platforms (e.g., airborne, space, and ground vehicles). Particularly with airborne platforms, where movement of aircraft can be rapid and unpredictable, it is critical that the pointing and tracking scheme provide accurate guidance for directing the data laser beams.
Consider a scenario in which two optical communication terminals whose relative positions may change are engaged in two-way communication (e.g., either one or both of the terminals are mobile). In each terminal, one option for determining the angular direction of the other terminal (i.e., the far-end terminal) is to split off a portion of the data signal (e.g., a laser beam) received from the far-end terminal and determine the angle of arrival of the split-off data signal. This approach has a number of disadvantages. The received signal power must be split between two detectors, one for detecting the pointing angle and one for receiving the data. By using a portion of the received data signal for angular position detection, only the remaining portion of the received data signal is available for reception of the data, thereby reducing the signal power at the receiver and reducing the maximum operating range of the system. Moreover, it is desirable to minimize the beamwidth of the data signal in order to maximize signal strength and operating range. Given the limited angular extent of the data signal, initial acquisition of a remote terminal is difficult with the data signal. Likewise, once a communication link has been established between terminals, it may be difficult for the terminals to continuously track each other using narrow data signal laser beams, since either terminal can fairly quickly move out of the beam when the relative angular direction of the terminals is changing rapidly.
Another option for determining the angular direction of a far-end terminal is to separately generate and transmit both a data signal and a beacon signal. The beacon signal can have a wider beamwidth, which is more suitable for acquisition and tracking. However, if two separate signals are created, the size, weight, and power of the system is typically doubled. Further, the two signals must be combined together at high power which is typically done in free space using costly optics that require careful alignment. Accordingly, there remains a need for a system capable of generating two optical signals, such as data and beacon signals in an optical communication system, without significantly increasing the size, weight, and power of the system relative to a single signal system and without diminishing signal power.